The electrochemical cell may be in open communication with the atmosphere or sealed from the atmosphere. A cell which is sealed from the atmosphere may comprise a valve which is dimensioned such that it becomes operative at a previously defined pressure.
In a rechargeable cell of the sealed type, the electrochemically active part of the positive electrode consists of, for example, nickel hydroxide, silver oxide or manganese oxide. Nickel hydroxide is generally preferred for practical reasons.
An electrolyte is used in the cell, which generally consists of an aqueous solution of one or more alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, having a pH value exceeding 7.
The cell may further comprise a separator which separates the electrodes electrically but permits ion and gas transport. Said separator may be composed of synthetic resin fibers (woven or not woven), for example, of polyamide fibers or polypropylene fibers.
Such an electrochemical cell is described in U.S. Pat. No. 4,487,817. In said patent, an electrochemically active material of the compositional formula AB.sub.m C.sub.n is used, where A denotes, for example, a rare earth metal, B denotes a mixture of elements, for example, a mixture of Ni and Co, and C consists of Al, Cr and/or Si. The value of n ranges between 0.05 and 0.6. The value of m+n is selected such that the material has the CaCu.sub.5 -structure. The limits of m+n depend on the choice of the elements A, B and C. The value of m+n always ranges between 4.8 and 5.4, mostly close to 5.0. Such intermetallic compounds having a CaCu.sub.5 -structure exhibit the property that large quantities of hydrogen can be stored therein by means of the formation of hydrides. Said known compounds are prepared by cooling a melt and are stable at room temperature. The materials in accordance with said United States Patent have a great stability, so that the electrochemical cell has a long life cycle. In comparison with a compound such as LaNi.sub.5, a part of the initial storage capacity has been sacrificed for this purpose. A further disadvantage of these known materials is that the reactivity is smaller than that of LaNi.sub.5 , as a result of which the loadability of the cell is smaller too, in particular at low temperatures.